diff --git a/src/main_udt.c b/src/main_udt.c
index f13d4dd..385f3f5 100644
--- a/src/main_udt.c
+++ b/src/main_udt.c
@@ -70,6 +70,11 @@
 	msc_send_direct(&bsc, _msg);
 }
 
+void mtp_link_set_forward_isup(struct mtp_link_set *set, struct msgb *msg, int sls)
+{
+	LOGP(DINP, LOGL_ERROR, "ISUP is not handled.\n");
+}
+
 void mtp_linkset_down(struct mtp_link_set *set)
 {
 	set->available = 0;